Passive optical network system

ABSTRACT

A system for providing bi-directional RF services over a point-to-multipoint Passive Optical Network (PON). A system that can transport upstream RF signals generated by devices such as a set top box or a cable modem, through a passive Optical Network while simultaneously supporting downstream RF video and bi-directional base-band services on the PON.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/116,320, filed May 26, 2011, entitled “Passive Optical NetworkSystem,” which is a continuation of U.S. patent application Ser. No.11/763,802, filed Jun. 15, 2007, entitled “Passive Optical NetworkSystem for the Delivery of Bi-Directional RF Services,” both of whichare hereby incorporated herein by reference in their entirety.

I. FIELD OF THE INVENTION

The invention broadly relates to broadband telecommunications systemsand particularly to those employing point-to-multipoint Passive OpticalNetworks (PON).

II. BACKGROUND

Currently there are broadband service providers deployingpoint-to-multipoint passive optical network systems to provide voice,data, and video services to customers. There are manypoint-to-multipoint PON technologies available today including BroadbandPON (BPON), Gigabit Ethernet PON (GEPON), and Gigabit PON (GPON).Standards bodies such as the International Telecommunication Union (ITU)and Institute of Electrical and Electronics Engineers (IEEE) havereleased standards for PON systems.

Systems based on point-to-multipoint passive optical network (PON), (seeFIG. 1) generally comprise an Optical Line Terminal (OLT) or OpticalLine Termination (OLT) connected via fiber to a 1:n passive opticalsplitter, which in turn is connected to a plurality of Optical NetworkUnits (ONUs) or Optical Network Terminals (ONTs). Optical Line Terminaland Optical Network Unit is the preferred naming convention for IEEEbased PON, Optical Line Termination and Optical Network Terminal is thepreferred naming convention in ITU 984.x PON. This invention isindependent of the specific PON technology used at the OLT and ONU/ONT.For simplicity this specification will use the term Optical LineTerminal (OLT) and Optical Network Unit (ONU) to represent the typicalelements of the PON system. The OLT contains an Optical Transmitter, anoptical receiver, and a Wavelength Division Multiplexer (WDM). TheOptical Transmitter transmits data downstream to the ONUs on an opticalwavelength. The Optical Receiver receives data upstream on an opticalwavelength from the ONUs. A Wavelength Division Multiplexer (WDM) istypically used to separate the optical wavelengths.

The ONUs contain an Optical Transmitter which transmits upstream data onan optical wavelength to the OLT, and an Optical Receiver to receivedownstream data on an optical wavelength from the OLT. As with the OLT,a WDM is typically used to separate the optical wavelengths. Data isbroadcast downstream from the OLT and appears at all ONUs via theoptical splitter. In the upstream direction, the ONUs useTime-Division-Multiple-Access (TDMA) to send upstream data to the OLT.Each ONU is assigned a timeslot to send its upstream data to the OLT.This insures that signals from multiple ONUs do not collide at theupstream port of the optical splitter. This type of ONU that operates attwo wavelengths will be referred to as a two-wavelength ONU.

The PON systems mentioned above operate at two wavelengths and aretypically used to provide data services such as web browsing, Voice overIP (VoIP). and IP video. These services are modulated on opticalwavelengths as base-band digital signals and will be referred to fromnow on as base-band services. In addition to these services, some PONsystems also provide an RF video service that is similar to a cable TVservice. In a typical scenario, this service includes several RFchannels that occupy a RF frequency spectrum from 50 to 870 MHz. Some ofthese channels are analog video channels that use a modulation techniquecalled Amplitude Modulated Vestigial Side Band (AM-VSB), while somechannels are digital channels that use QAM (Quadrature AmplitudeModulation). This RF frequency band comprising analog and digitalchannels is modulated into an optical carrier at wavelength λ3 andinserted into the PON using a WDM as shown in FIG. 2. At the subscriberside, the wavelength is separated using a WDM and converted into RF fordistribution within the customer premises. This type of ONU thatoperates at three wavelengths will be referred to as a three-wavelengthONU.

In the descriptions of various figures, we refer to a two-wavelength ONUor a three-wavelength ONU. In general, these ONUs can also be referredto as multi-wavelength capable ONUs.

The three-wavelength system in FIG. 2 was an improvement over thetwo-wavelength system shown in FIG. 1 because it gave the systemoperator the ability to provide another revenue generating service.However, the system in FIG. 2 has certain limitations that prevent theSystem Operator from providing advanced video services such as Video onDemand (VoD) and Network Digital Video Recorder (Network DVR). Theseservices require a set top box at the customer premises that cancommunicate upstream the customer's requests such as movie selection,channel selection, pause, fast forward, etc. These upstream RF signalstypically occupy a frequency band from 5 to 42 MHz. When the customeractivates the set top box, typically via a remote control, to requestmovies, or to pause a movie that is currently playing, this request ismodulated into a RF carrier by the set top box and sent upstream to theset top box controller that processes the request. The system in FIG. 2is not capable of transporting these types of upstream RF signals.

It will be appreciated that a set top box isn't the only device at thecustomer premises that can generate upstream RF signals. Upstream RFsignals can also be generated by cable moderns or other devices thatoffer other services.

In summary, what is needed is a system that carries upstream RF set topbox and cable modem information while simultaneously supportingdownstream RF video and bi-directional base-band services on the PON.

III. SUMMARY OF THE INVENTION

Accordingly, the objective of the present system and apparatus is toprovide a system that can transport upstream RF signals generated bydevices such as a set top box or a cable modern, through a passiveOptical Network while simultaneously supporting downstream RF video andbi-directional base-band services on the PON. Such a system can beconfigured in three ways as described below:

In the first embodiment of the system (see FIG. 3) node 350 operating atoptical wavelength λ4 is used to transmit upstream RF signals. Thissignal is de-multiplexed at the central office or hub by a WDM(Wavelength Division Multiplexer) 335 and routed to an RF OpticalReceiver. The ONU, which is connected to the node 350 via port 352, isused to provide bi-directional base-band services as well as adownstream RF service.

In the second embodiment of the system (see FIG. 4) node 445 performsthe function of both transmitting the upstream RF signals as well asreceiving the downstream RF signals. The ONU that is connected to port452 of node 445, provides bi-directional base-band services.

In a third embodiment of the system (see FIG. 5), node 545 is connectedto one downstream port of an optical splitter and is used for providingdownstream and upstream RF services while an ONU on a differentdownstream port of the same splitter is used to provide base-bandservices.

In all three cases, four wavelengths co-exist on the PON between the Hubor central office and the customer premises.

IV. BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1 and FIG. 2 are diagrams illustrating conventional PON systems.

FIG. 3 depicts a PON system in accordance with an embodiment of thedisclosures made herein, which utilizes a node with an opticaltransmitter to transmit upstream RF signals and route other wavelengthsassociated with the PON to a Three-Wavelength ONU.

FIG. 4 depicts a PON system in which a node with an optical transmitterand an optical receiver is used to transmit and receive upstream anddownstream RF signals while routing other optical signals associatedwith the PON to a Two-Wavelength ONU.

FIG. 5 depicts a PON system in which a node attached to one downstreamport of a splitter is used to transmit and receive RF signals while aTwo-Wavelength ONU attached to another downstream port of the samesplitter is used to receive and transmit base-band services.

FIG. 6 depicts three locations served by a PON, wherein one location isprovided both bi-directional RF services and base-band services, while asecond location is provided base-band services, and the third locationis provided bi-directional RF services.

V. DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1. illustrates a typical PON system comprising an OLT, an opticalsplitter, and a plurality of ONUs connected to the OLT via the opticalsplitter. The OLT is generally located in a Hub or Central Office andthe ONU is typically located at the customer premise. The customerpremise may be a single family home, apartment building, hotel, place ofbusiness, or other structure where telecom equipment may be located. TheONU is typically installed inside the home or building or attached tothe outside of the home or building.

The OLT 100 typically includes an optical transmitter 105 operating atan optical wavelength λ1 and an optical receiver 110 receiving anoptical wavelength λ2 from a plurality of Two-Wavelength ONUs 130 a-130n. In the downstream direction, λ1 is multiplexed into optical fiber 120by WDM 115. It then passes through optical splitter 125, which splitsthe signal into a plurality of optical fibers 128 a-128 n. The opticalfibers 128 a-128 n are connected to a plurality of Two-Wavelength ONUs130 a-130 n. The ONU comprises a WDM (135 a-135 n) that de-multiplexesλ1 and routes it to optical receiver (155 a-155 n).

In the upstream direction, ONUs 130 a-130 n typically include opticaltransmitters 150 a-150 n that transmit upstream signals at λ2. Thesesignals are multiplexed into fibers 128 a-128 n by WDMs 135 a-135 n.These signals then pass through optical splitter 125, optical fiber 120and are de-multiplexed by WDM 115, and are input into optical receiver110. The ONUs 130 a-130 n comprise optical transmitters 150 a-150 ntransmit their signals using a Time Division Multiple Access protocol,where each ONU is assigned a time slot in which it sends its data to theOLT 100. This ensures that multiple ONUs don't transmit upstream at thesame time, thus preventing interference at the optical receiver 110.

FIG. 2 illustrates a typical PON system as known in prior art comprisingan OLT, an RF Optical Transmitter, a WDM, an optical splitter, and aplurality of ONUs connected to the OLT via the optical splitter, wherethe RF optical transmitter is used to insert RF services into the PON inthe downstream direction only. Downstream RF services are modulated byRF Optical Transmitter 220 operating at λ3 and multiplexed into the PONby WDM 225. λ3 then passes through optical fiber 230, and opticalsplitter 235, which splits the signal, which is input into a pluralityof optical fibers 240 a-240 n. The optical fibers 240 a-240 n areconnected to a plurality of Three-Wavelength ONUs 250 a-250 n. The ONUs250 a-250 n contain WDMs 260 a-260 n that de-multiplex λ3 and route itto Downstream RF Optical Receivers 250 a-250 n. The downstream RFOptical Receivers convert λ3 back to electrical RF for distributionwithin the customer premise.

FIG. 3 illustrates an embodiment of the invention comprising a node 350which transmits upstream RF signals at optical wavelength λ4. ONU 355 isused to provide base-band services and the downstream RF service. Forthe sake of clarity, the following definitions will be used:

-   -   λ1 is the optical wavelength transmitted from OLT 300 in the        downstream direction.    -   λ2 is the optical wavelength transmitted by ONU 355 in the        upstream direction.    -   λ3 is the optical wavelength transmitted by the Downstream RF        Optical Transmitter 320 in the downstream direction.    -   λ4 is the optical wavelength transmitted by node 350 in the        upstream direction.

Upstream RF signals from such devices as cable modems or set top boxestravel through coaxial cable 392 and are de-multiplexed from thedownstream signals by RF diplexer 390. An RF diplexer is a device thatseparates RF frequencies. Upstream RF Optical Transmitter 365 modulatesthese signals into optical wavelength λ4. λ4 is routed through WDM 360to port 351 of the node 350, and then to WDM 335 in the hub or centraloffice via optical fiber 347, optical splitter 345 and optical fiber340. WDM 335 de-multiplexes λ4 and routes it to the RF Optical Receiver325.

Downstream wavelengths λ1 and λ3 are multiplexed into optical fiber 340by WDM 335. λ1 and λ3 are transported to port 351 of node 350 viaoptical fiber 340, optical splitter 345 and optical fiber 347. Insidenode 350, λ1 and λ3 are routed from port 351 to port 352 by WDM 360.These two wavelengths are then transported to Three-Wavelength ONU 355which is connected to Port 352 of node 350. The RF output ofThree-Wavelength ONU 355 in multiplexed onto coaxial cable 392 viadiplexer 390. Coaxial cable 392 is connected to devices such as cablemodems and set top boxes or other devices that receive and/or generateRF services. λ2 from Three—Wavelength ONU 355 is routed from port 352 toport 351 of node 350 via WDM 360. λ2 is then transported to WDM 335 viaoptical fiber 347, optical splitter 345, and optical fiber 340. WDM 335de-multiplexes λ2 and routes it to OLT 300. It will be appreciated thatthe exact placement of WDM 360 is not important. For example, WDM 360could be placed outside node 350 and still provide the same function.Likewise, it can be placed inside Three Wavelength ONU 355 and stillprovide the same functionality. Similarly, the node 350 and the diplexer390 can be placed inside ONU 355 and provide the same functionality. Itwill also be appreciated that although the various WDMs, opticaltransmitters and optical receivers are shown as separate devices, theycould easily be integrated together. For example, WDM 360 can becombined with the laser of RF Optical Transmitter 365 in a singlepackage. It will also be appreciated that optical splitter 345 is shownas a single device for the sake of clarity. In practice, there can beseveral 1:n optical splitters connected in cascade. For example, opticalsplitter 345 can be a single 1:32 splitter or the same function can beperformed by a 1:4 splitter connected to four 1:8 splitters.

It will also be appreciated that Optical Splitter 345 doesn't need to beinstalled for the system to function. Fiber 340 can be connected to port351 of Node 350 to provide the same functionality.

FIG. 4 illustrates another embodiment of the invention that includes anode 445 that comprises an RF Optical transmitter 460, an RF Opticalreceiver 465, RF diplexer 470, WDM 450 and WDM 455. Node 445 providesthe upstream and downstream RF services while Two—Wavelength ONU 475provides base-band services.

The two downstream wavelengths λ1 and λ3 are multiplexed on fiber 435 byWDM 430. They are routed to port 451 of node 445 via optical splitter440 and optical fiber 447. Inside node 445, WDM 450 routes λ3 to WDM455, which de-multiplexes λ3 and routes it to downstream RF opticalreceiver 465. RF optical receiver 465 converts λ3 back into electricalRF and routes it to RF diplexer 470 which multiplexes the downstream RFsignals on coaxial cable 492 for distribution within the customerpremises.

λ1 is routed from port 451 to port 452 of node 445 via WDM 450 and thento Two-Wavelength ONU 475 connected to port 452 of node 445. λ2 fromTwo-Wavelength ONU 475 is routed from port 452 of Node 445 to port 451of Node 445 via WDM 450.

The upstream RF signals on coaxial cable 492 are de-multiplexed by RFdiplexer 470 and routed to upstream RF optical transmitter 460 operatingat λ4. λ4 is then routed to port 451 of node 445 through WDM 455 and WDM450. From port 451, the two upstream wavelengths λ2 and λ4 aretransported to WDM 430 via fiber 447, optical splitter 440, and opticalfiber 435. WDM 430 de-multiplexes λ2 and λ4 and routes it to OLT 400 andupstream RF optical receiver 425 respectively. RF optical Receiver 425converts λ4 back into RF.

It will be appreciated that upstream and downstream RF services can beprovided without installing OLT 400 and Two-Wavelength ONU 475. Thesystem can be deployed with only two wavelengths, λ3 and λ4 that providebi-directional RF services. The OLT and the ONU can be added at a laterdate to increase bandwidth.

FIG. 5 illustrates another embodiment of the invention comprising a node545 that provides bidirectional RF services and is attached to one portof optical splitter 540. Two-Wavelength ONU 575 attached to another portof optical splitter 540 provides base-band services. λ3 transmitted bythe RF Optical Transmitter 520 and λ1 transmitted by OLT 500 are routedto node 545 via WDM 530, fiber 535, optical splitter 540 and opticalfiber 543 a. At node 545, WDM 555 routes λ3 to RF Optical Receiver 565.The RF output of RF Optical Receiver is multiplexed onto coaxial cable592 via diplexer 570. Coaxial cable 592 is connected to devices such ascable modems and set top boxes or other devices that receive and/orgenerate RF services. WDM 555 also prevents λ1 from reaching either RFoptical transmitter 560 or RF optical receiver 565. This prevents anyinterference between λ1 and the bi-directional RF services provided bynode 545.

The upstream RF signals transmitted by devices such as cable modems andset top boxes are de-multiplexed by RF Diplexer 570 and converted intooptical wavelength λ4 by RF optical transmitter 560. λ4 is routed to WDM530 via WDM 555, optical fiber 543 a, optical splitter 540, and opticalfiber 535. WDM 530 de-multiplexes λ4 and routes it to RF Opticalreceiver 525.

λ1 and λ3 multiplexed by WDM 530 are also routed to Two-Wavelength ONU575 via optical fiber 535, optical splitter 540 and optical fiber 543 n.The internal WDM of ONU 575, prevents λ3 from interfering with thebi-directional base-band services provided by Two-Wavelength ONU 575.

It will be appreciated that bi-directional RF services can be providedwithout installing either OLT 500 or ONU 575. Likewise, base-bandservices can be provided without installing either downstream RF opticaltransmitter 520, upstream RF optical receiver 525 or node 545.

It will also be appreciated that for the sake of clarity, FIG. 5 onlydepicts two fibers connected to optical splitter 540. In practice,several fibers can be routed from optical splitter 540 to various homesand businesses. For example, fiber 543 a can be routed to a home toprovide bi-directional RF services. At the same time, fiber 543 n can berouted to a business to provide base-band services. In this manner,several fibers can be routed to different locations to provide either RFor base-band services. All four wavelengths are present on the PON butthe device at the customer premise selects only those wavelengthsrequired to deliver the requested services. The other wavelengths areprevented from interfering with the requested services by WDM 555 and/orWDM 580.

Another embodiment of such a system is shown in FIG. 6. Location 680,connected to the PON via optical fiber 641 a comprises Two-WavelengthONU 675 which provides base-band services and node 665 which provideboth bi-directional RF services. Location 681, connected to the PON viaoptical fiber 641 b has a Two-wavelength ONU 676 that provides base-bandservices, while location 682, connected to the PON via optical fiber 641c has node 677 that provides bi-directional RF services. In this mannereither bi-directional RF services, base-band services, or a combinationof these services can be provided at various locations served by thePON.

We claim:
 1. A node, consisting of: a first upstream port for couplingto an optical fiber carrying optical signals in a bidirectional passiveoptical network; a second upstream port for coupling a downstreamoptical signal to an Three-Wavelength Optical Network Unit (ONU) thathandles a plurality of wavelengths; an RF optical transmitter forreceiving an upstream Radio Frequency (RF) signal from a diplexer andtransmitting an upstream RF optical signal over the bidirectionalpassive optical network; and a Wavelength-Division-Multiplexer (WDM),coupled to the RF optical transmitter and the first upstream port,operative to multiplex the upstream RF optical signal with otherupstream optical signals onto the bidirectional passive optical network.2. The node of claim 1, wherein the upstream RF optical signal comprisesvideo content.
 3. The node of claim 1, wherein the upstream RF opticalsignal is generated by a set top box.
 4. The node of claim 1, whereinthe upstream RF optical signal is generated by a cable modem.
 5. Thenode of claim 1, wherein the WDM is coupled to the second upstream port,and wherein the second upstream port is configured to receive anupstream optical signal from the Three-Wavelength ONU.
 6. The node ofclaim 1, wherein the other upstream optical signals include at least anupstream optical signal from the Three-Wavelength ONU.
 7. The node ofclaim 1, wherein the WDM is adapted to route the downstream opticalsignal from the first upstream port to the second upstream port.
 8. Thenode of claim 5, wherein the WDM is adapted to route the upstreamoptical signal of the Three-Wavelength ONU from the second upstream portto the first upstream port.
 9. The node of claim 1, wherein the upstreamRF optical signal and downstream optical signal each comprise videocontent.